Process for stabilizing autocatalytic metal plating solutions



United States Patent 3,403,035 PROCESS FOR STABILIZING AUTOCATALYTIC METAL PLATING SOLUTIONS Frederick W. Schneble, Jr., Oyster Bay, Rudolph J. Zeblisky, Hauppague, John F. McCormack, Roslyn Heights, and John Duff Williamson, Miller Place, N .Y., assignors to Process Research Company, Glen Cove, N.Y., a partnership of New York No Drawing. Filed June 24, 1964, Ser. No. 377,471 11 Claims. (Cl. 1061) The present invention relates to electroless or autocatalytic metal plating solutions and more particularly to improving the stability and useful life of such solutions, and to autocatalytic metal plating solutions having enhanced stability and relatively long useful lives.

Chemical plating solutions for continuously depositing metals or alloys by autocatalytic chemical reduction of metal ions dissolved in the solutions and in contact with a catalytic surface of an article to be plated are well known. Such solutions, which do not utilize electricity, are sometimes referred to in the art as electroless metal plating solutions, to distinguish them from electroplating solutions, which do require the use of electricity. Electroless metal deposition is also to be distinguished from displacement metal plating of the type described in Metals Finishing Guide Book, 27th Edition, 1959, pages 469 et seq., and metal mirroring procedures, wherein the metal plating desired and achieved is only a few millionths of an inch in thickness.

Typically autocatalytic or electroless metal deposition solutions comprise an aqueous solution of water soluble salt of the metal or metals to be deposited, a reducing agent for the metal cations, and a complexing or sequestering agent for the metal cations. The function of the complexing or sequestering agent is to form a water soluble complex with the dissolved metallic cations so as to maintain the metal in solution. The function of the reducing agent is to reduce the metal cation to metal at the appropriate time, as will be made more clear hereinbelow.

For proper operation, the pH of the autocatalytic met-a1 solution will ordinarily be controlled. Solution pH will depend upon the nature of the metallic salt, the sequestering agent and the reducing agent. Depending upon the ingredients, the pH may range from strongly acid to strongly alkaline, e.g., from pH 1 to 14, and usually from pH 4 to 14.

The goal in operating electroless metal deposition solutions is to balance the ingredients so that metallic deposition will occur only at the interface formed between the solution and a catalytic surface to be metallized.

As a practical matter, however, it is not possible to completely maintain the metal in solution. There is, in other words, an inherent tendency in electroless metal deposition baths for precipitates of the metal to form. The metallic precipitate constitutes a decomposition product of the plating bath, and is particularly objectionable in that it causes the desirable deposit to be coarse, rough and frequently porous. Additionally, the fine solid particles of the metallic precipitate, once formed, become suspended in the plating baths or adhere to the walls of the plating vessel, and act as nuclei to initiate the formation of additional precipitate, thereby further enhancing the tendency of the bath to decompose.

Because of their inherent tendency to form metallic precipitates, all electroless metal solutions have a finite life. It would be very desirable to extend this life, and the principal object of this invention is to do so.

According to this invention, it has been discovered that the useful life of autocatalytic metal plating solutions may be enhanced by adding to the solution a small effective amount of a cyanide compound.

Patented Sept. 24, 1968 As the cyanide compound may be mentioned alkali cyanides, such as sodium and potassium cyanide, and nitriles such as alpha-hydroxy nitriles, e.g., glycolnitrile and lactonitrile.

The amount of cyanide compound will ordinarily range between about 5 micrograms and 500 milligrams per liter.

Care should be employed in adding the cyanide compounds to insure that an excess is not employed. Too much cyanide compound is deleterious and will cause the solution to cease functioning. Within the above range, there will be used a small effective amount of the cyanide compound which is low enough to permit the solution to function.

The invention is generally applicable to autocatalytic metal deposition solutions for copper, nickel, cobalt, cadmium, tin, and similar metals. In such baths, the sulfates, chlorides, acetates, and nitrates of such metals are ordinarily utilized as the water soluble salts. Other water soluble salts of the indicated metals may however be used, the choice being primarily a question of economics.

The reducing agents in such metal deposition baths include borohydrides, amine boranes, hydrazines, hypophosphites, hydrosulfites, hydroxyl amines, formaldehyde, and the like.

The borohydride reducing agent may consist of any water soluble borohydride having a good degree of solubility and stability in aqueous solutions. Sodium and potas sium borohydrides are preferred. In addition, substituted borohydrides in which not more than 3 hydrogen atoms of the borohydride ion have been replaced can be utilized. Sodium trimethoxy borohydride, NaB(OCH H, is illustrative of the compounds of this type.

Also may be mentioned ammonium borohydride and the amine boranes, such as isopropylamine borane, and dimethyl amine borane.

Among the hydrazines may be mentioned water soluble salts of hydrazine, such as the hydrazine sulfates, e.g., monohydrazine sulfate, di-hydrazine sulfate, hydrazine disulfate, and the like. Because of its greater solubility in Water, dihydrazine sulfate is preferred.

Among the hypophosphite and hydrosulfite reducing agents, the alkali metal, e.g., sodium, potassium and ammonium salts are preferred.

In addition to formaldehyde, polymers of the formaldehyde, e.g., paraformaldehyde, may be used as the reducing agent. Also suitable as the reducing agent is alpha-trioxymethylene.

The sequestering or complexing agent will be selected to form a strong complex with the metal ions to prevent the precipitation of metal or metallic salts. The complexing agent selected should also be capable of forming a metal complex which is soluble in the plating solution, and also which is sufliciently stable so that it will not react with the reducing agent in the main body of the plating solution, but only at or in the near vicinity of the catalytic surface.

The complexing or sequestering agents suitable for use in accordance with this invention include ammonia and organic complex-forming agents containing one or more of the following functional groups: primary amino group .(NH secondary amino group NH), tertiary amino group N), imino group (=NH), carboxy gluconates, and triethanolamine are preferred as complexing agents, but commercially available glucono 6- lactone and modified ethylenediamineacetates are also useful, and in certain instances give even better results than the pure sodium ethylenediaminetetraacetates. One such material is N hydroxyethylethylenediaminetriacetate.

In preparing the electroless metal deposition baths, it is desirable to combine the bath ingredients in such a manner as to avoid reaction between the soluble metal salt and the reducing agent.

It is therefore preferred to first add the complexing agent to the aqueous solution of the metal salt to form the water soluble complex of the metal cations. If the reducing agent is added before the metal complex is formed, there will be a tendency for it to react instantaneously with the metal salt to precipitate the metal or a salt of the metal.

The pH should also be adjusted to the proper value prior to addition of the reducing agent. Here again, failure to regulate the pH initially may lead to spontaneous decomposition of the reducing agent. In view of the foregoing, it will be appreciated that ordinarily it is preferable to add the reducing agent last.

Catalytic surfaces which may be plated with the baths of this invention include metals such as nickel, cobalt, iron, steel, palladium, platinium, copper, brass, manganese, chromium, molybdenum, tungsten, titanium, tin, silver, including mixtures thereof, and oxides thereof. All such metals are catalytic to the reduction of the metal cations dissolved in the solutions described.

Non-metallic materials such as glass, ceramic, and various plastics are, in general, non-catalytic. However, the surfaces of such non-catalytic materials can be rendered catalytic by producing a film or particles of one of the catalytic materialst thereon. This can be accomplished by a variety of techniques known to those skilled in the art. One suitable sensitization procedure involves dipping noncatalytic materials in an acidic solution of stannous chloride, washing with water, and then contacting the treated material with an acidic solution of a precious metal salt, e.g., palladium chloride. A mono-layer of precious metal is thus produced on the surface of the non-catalytic material, which mono-layer is catalytic to reduction of the metal ions in the electroless metal plating solutions described herein.

Alternatively, such materials as glass, ceramic, and plastic may be sensitized or rendered catalytic by treatment with an acidic aqueous solution containing, in combination, stannous tin ions and precious metal ions.

A further sensitization procedure involves adhering to the normally non-catalytic surface, finely divided particles of metals or metal oxides which are catalytic to electroless deposition.

The term catalytic surface as used herein refers to the surface of any article composed of the catalytic materials described hereinabove or covered therewith or to the surface of a non-catalytic material which has been sensitized by producing a film or particles of said catalytic materials on its surface.

The baths of this invention will deposit metal electrolessly on the catalytic surfaces of metals, and non-metals, such as paper, glass, ceramic, synthetic resins and plastics, including but not limited to silicones, phenolics, styrenes, acrylics, vinyl chlorides, nylon, mylar, acrylonitrilebutadiene-styrene, and the like.

The following examples are intended to illustrate the manner in which the present invention may be carried out.

Example 1 A bath corresponding to the following formula was prepared:

Copper sulfate moles/l 0.05 Diethylenetriamine pentaacetate do 0.05 Sodium borohydride do 0.009

Sodium cyanide do 0.00 pH 13 Temperature C 25 As a control, a bath having the same composition but without the sodium cyanide was prepared.

The control bath decomposed spontaneously after standing for /2 hour. The bath containing 0.008 mole per liter of sodium cyanide had a useful copper plating life of greater than 4 hours.

Example 2 A bath corresponding to the following formula was prepared:

Copper sulfate moles/l 0.05 N-hydroxy-ethylethylenediaminetriacetate do 0.115 Sodium cyanide -do 0.0016 Sodium borohydride do 0.008 pH 13 Temperature C 25 As a control, a bath having the same composition but without the sodium cyanide was prepared.

The control bath decomposed spontaneously after standing /2 hour, The bath containing 0.0016 mole per liter of sodium cyanide had a useful copper plating life of 20 hours.

Example 3 A bath corresponding to the following formula was prepared:

Copper sulfate moles/l 0.05 N-hydroxyethylethylenediaminetriacetate do 0.125 Sodium cyanide -do 0.001 Hydrazine sulfate do 0.019 pH 9.8 Temperature C 43 Example 4 A bath corresponding to the following formula was prepared:

Copper oxide "moles/L- 0.05 Malic acid do 0.30 Sodium cyanide do 0.00001 Sodium hypophosphite do 0.5 pH 4-5 Temperature C 80 As controls, a bath having the same composition but without the sodium cyanide, and a bath having the same composition but containing 0.0001 mole/l. of sodium cyanide were prepared.

The control bath without cyanide decomposed spontaneously forming a reddish precipitate. The bath containing 0.0001 mole per liter of sodium cyanide did not deposit copper. The bath with 0.00001 mole per liter of sodium cyanide deposited a bright copper film.

The quantities of the various ingredients in the baths of this invention are subject to wide variation. Typically, however, the bath constituents will be as follows:

Water soluble metal salt 0.002 to 0.60 mole/l. Reducing agent 0.0002 to 2.5 moles/l. Complexing agent 0.7 to 10 times the moles of metal salt. Cyanide compound 0.00001 to 0.06 mole/l.

The amount of sequestering agent to be added to the plating solution depends upon the nature of the sequestering agent and the amount of the metal salt present in the bath. In alkaline solutions, the preferred ratio of the metal salt to complexing agent lies between about 1:1 and 1:10. A small excess of the sequestering agent, based upon the metal salt, generally is advantageous.

It should be understood that as the baths are used up in plating, the metal salt, and the reducing agent may be replenished from time to time, and also that it may be advisable to monitor the pH, and the cyanide content of the bath, and to adjust them to their optimum value as the bath is used.

For best results, surfactants in an amount of less than about 5 grams per liter are added to the baths disclosed herein. Typical of suitable surfactants are organic phosphate esters, and oxyethylated sodium salts. Such surfactants may be obtained under the trade names Gafac RE 610 and Triton QS-lS, respectively.

The baths are ordinarily used at temperatures between 25 and 90 C., although they may be used at lower temperatures or at even higher temperatures. As the temperature is increased, it is usual to find that the rate of plating is increased.

The process of this invention permits metal to be deposited on practically any conceivable substratum and to practically any thickness desired.

Metallized surfaces produced by utilizing the autocatalytic metal deposition solutions of this invention are useful as ornamental designs, markings, and the like.

' Similarly, using these baths, metal may be deposited in pre-determined patterns on insulating substrata and serve as electrical conductors. Electrically conductive circuit patterns, e.g., printed circuits, may thus be selectively plated on the activated areas of an inexpensive sheet of a wide variety of insulating material.

The invention in its broader aspects is not limited to the specific steps, processes and compositions described but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. A process for increasing the useful life of autocatalytic metal plating solution used to electrolessly deposit metal onto catalytic surfaces, in which the solution comprises water, a metal ion selected from the group consisting of cobalt, nickel and tin, a complexing agent and a reducing agent for said ion, said process comprising maintaining in said solution a small eflective amount of a water-soluble cyanide compound in a concentration of 5 micrograms to 500 milligrams per liter.

2. A process according to claim 1 in which the reducing agent is a member selected from the group consisting of borohydrides, amine boranes, hydrazines, hypophosphites, hydrosulfites, hydroxyl amines and alpha-trioxymethylene.

3. A process according to claim 1 in which the complexing agent is an organic complex forming compound having at least one functional group selected from the group consisting of amino, imino, carboxy and hydroxy radicals.

4. A process according to claim 1 in which the watersoluble cyanide compound is an alphahydroxynitrile.

5. A process according to claim 4 in which the alphahydroxynitrile is glycolnitrile.

6. A process according to claim 4 in which the alphahydroxynitrile is lactonitrile.

7. A process according to claim 1 in which the watersoluble cyanide compound is sodium cyanide.

8. A process according to claim 1 in which the watersoluble cyanide compound is potassium cyanide.

9. A process according to claim 1 in which the metal of the autocatalytic metal plating solution is cobalt.

10. A process according to claim 1 in which the metal of the autocatalytic metal plating solution is nickel.

11. A process for electrolessly depositing a metal onto a catalytic surface which comprises immersing the catalytic surface in an autocatalytic metal plating solution comprising water, a water-soluble salt of a metal selected from the group consisting of cobalt, nickel and tin, a complexing agent for said metal capable of forming a stable water-soluble complex, a reducing agent for said metal and a water-soluble cyanide compound as a stabilizing agent in a concentration of from about 5 micrograms to 500 milligrams per liter.

References Cited UNITED STATES PATENTS 3,096,182 7/1963 Berzins 106--1 3,046,159 7/ 1962 Brookshire 106-1 X 3,063,850 11/1962 Mikulski 1061 3,095,309 6/ 1963 Zeblisky et al. 106-1 3,119,709 1/ 1964 Atkinson 106-1 JAMES A. SEIDLECK, Primary Examiner.

L. B. HAYES, Assistant Examiner. 

1. A PROCESS FOR INCREASING THE USEFUL LIFE OF AUTOCATALYTIC METAL PLATING SOLUTION USED TO ELECTROLESSLY DEPOSIT METAL ONTO CATALYTIC SURFACES, IN WHICH THE SOLUTION COMPRISES WATER, A METAL ION SELECTED FROM THE GROUP CONSISTING OF COBALT, NICKEL AND TIN, A COMPLEXING AGENT AND A REDUCING AGENT FOR SAID ION, SAID PROCESS COMPRISING MAINTAINING IN SAID SOLUTION A SMALL EFFECTIVE AMOUNT OF A WATER-SOLUBLE CYANIDE COMPOUND IN A CONCENTRATION OF 5 MICROGRAMS TO 500 MILLIGRAMS PER LITER. 